It is generally known in colder climates that the insulating properties of window assemblies can be enhanced by providing more panes of glass with the panes being separated by an enclosed space. In addition to the construction of dual and triple pane windows, it is further known that providing additional panes of transparent plastic such as acrylic further enhances the insulation properties of the window assembly.
U.S. Pat. No. 4,194,331 by Gingle et al discloses one example of a window assembly in which an auxiliary pane is selectively attached to an existing pane of an existing window assembly so as to permit ready separation in warmer seasons and the like. A magnetic mounting strip is secured adhesively directly to the existing pane of glass such that a perimeter edge of the auxiliary pane can be magnetically coupled thereto when provided with its own magnetic mounting strip at the perimeter thereof. In this instance, the auxiliary pane must be very carefully cut with close tolerance to fit within the perimeter frame about the existing pane of the window assembly, otherwise a perimeter gap will result which permits leakage of heat therethrough and has an unpleasant and unfinished appearance. Accordingly an additional trim piece is required to be subsequently attached about the perimeter of the auxiliary pane to enclose the gap.
To overcome the disadvantages described above with regard to Gingle et al, it is also generally known in the prior art to make use of an L-shaped mounting member having a first flange fastened with screws to a surface of the perimeter frame which projects perpendicularly to the existing pane of the window assembly. The second flange of the mounting member is intended to lie substantially parallel to the existing pane of the window assembly about the perimeter of the existing plane to provide a mounting surface relative to which the auxiliary pane can be magnetically coupled. As the first flange is fastened directly to the perimeter frame of the window assembly, there is no problem with regard to a perimeter gap as in the prior art by Gingle et at; however, in this instance the perimeter frame of the existing window is relied on for positioning of the mounting surface relative to which the auxiliary pane is magnetically coupled. In practice, the resulting mounting surface about the perimeter of the window frame when fastening the first flange to the perimeter frame because the fasteners tend to distort the shape of the mounting members as they are penetrated therein, and because the existing window frame used for aligning the mounting member is rarely true and planar about the full perimeter thereof due to warping and the like. Accordingly, the magnetic mounting strip about the perimeter of the auxiliary pane commonly does not make sufficient contact with mounting surface defined by the mounting members about the perimeter of the window frame, resulting in failure of the auxiliary panes to remain properly attached in many instances.